This invention relates to sensors for controlling the transfer of energy from a power generator to a load so as to increase the efficiency of the transfer.
In many power conversion applications, transducers, functioning as power generators, are used which are responsive to recurring (oscillating) natural sources of energy (e.g., ocean waves, wind, eddies of water) for converting energy captured from these sources of energy into electrical energy.
In many applications, such as those involving extracting energy from ocean waves, it is necessary and/or desirable to sense the occurrence of at least one of the peak, amplitude and frequency of the ocean waves to optimize the transfer of energy from the ocean waves.
By way of example, a capacitive electric power generator may include a piezoelectric device functioning as a capacitive piezoelectric generator (PEG) which when subjected to mechanical stresses and strains produces an electrical signal. The electrical signals of one, or more, of these piezoelectric devices may be processed to produce electrical power which can be used to operate electrical/electronic devices and/or which can be part of an electrical power grid. Systems making use of piezoelectric devices to produce electrical power are shown, for example, in U.S. Pat. Nos. 5,552,656 and 5,703,474 which issued Sep. 3, 1996 and Dec. 30, 1997, respectively, and which are assigned to the assignee of the present application, and whose teachings are incorporated herein by reference.
Piezoelectric devices used as electric power generators are characterized by an inherent inefficiency in the transformation (xe2x80x9ccouplingxe2x80x9d) of the mechanical strains and stress into electrical charge. As a result, often only a small portion (e.g., approximately 10%) of the mechanical stress/strain applied to a piezoelectric device is available as electrical power when a constant load is applied to the piezoelectric device. It is therefore desirable to increase the efficiency with which the energy generated by a piezoelectric device is transferred to a load to compensate for, and overcome, the low xe2x80x9ccouplingxe2x80x9d factor of the piezoelectric devices.
A known method for increasing the efficiency of the transfer from the piezoelectric generator to a load includes forming a resonant circuit. This is shown, for example, in FIG. 1, which is a highly simplified block diagram representation of a prior-art piezoelectric electric power generator circuit. The stresses and/or strains applied to the piezoelectric device are provided by sources of energy (e.g., ocean waves, wind, eddies of water) which may vary slowly (e.g., a few cycles per second). Consequently, the piezoelectric devices may be operated at very low frequencies and the frequency of the electrical signals produced by these piezoelectric devices is also in the range of a few cycles per second. These low operating frequencies present significant problems to the efficient transfer of energy from the piezoelectric device to a load.
For example, it is difficult to form inductors and transformers of reasonable size and at a reasonable cost which can operate at those frequencies. Referring to FIG. 1, by way of example, note that the circuit includes a piezoelectric device 22 coupled by an inductor 16 to a load 27. The resonant frequency (fo) of the circuit may be expressed as fo=1/2xcfx80(LCp)0.5; where Cp is the capacitance of the piezoelectric device 22; and L is the inductance of inductor 16, with the value of L being selected to resonate with the capacitance of the piezoelectric device. [Note: for ease of explanation and discussion, the contribution of other capacitances in the circuit have been ignored in the specification and claims which follow]. The capacitance of Cp may be assumed to be in the range of 0.01 to 10 microfarads (10xe2x88x926 farads). Assume now that the frequency of the electric signal, produced by the piezoelectric device in response to the mechanical driving force, is in the range of 2 Hz. Then, in order to have a circuit that resonates at 2 Hz, an inductor 16 having a value in the range of 12,000 Henrys would be needed. An inductor of this value would be the size of a small room. In addition, direct electrical resonance is not practical because of the expected variability of the frequency due to the random nature of ocean waves.
As disclosed and claimed in co-pending applications titled xe2x80x9cApparatus And Method For Optimizing The Power Transfer Produced By A Wave Energy Converter (WEC)xe2x80x9d filed Aug. 6, 2001 and bearing Ser. No. 09/922877 and xe2x80x9cSwitched Resonant Power Conversion Electronicsxe2x80x9d bearing Ser. No. 09/933,158 and both assigned to the assignee of the present application, and the teachings of which are incorporated herein by reference, Applicants recognized that it is advantageous to selectively switch a load in circuit with the power generating device which is designed to resonate at a higher frequency than the frequency of the input force. However, it is important to determine the point at which the switching should take place.
Systems and circuits embodying the invention include means for sensing at least one of the peaks, amplitude and frequency of an oscillatory input force for controlling the transfer of energy collected by a transducer (power generator) to an associated load. In systems embodying the invention the power generator may be a transducer such as a piezoelectric device or a wave energy converter (WEC) or any like device responsive to an oscillatory input force for generating electrical energy.
In one embodiment of the invention, a power generating device captures energy at a low frequency rate of the input force. Sensors embodying the invention enable the collected energy to be extracted at a much higher frequency. Extracting the energy at a higher frequency enables the use of components, such as inductors, having reasonable values and sizes compared to the prior art systems. In accordance with the invention, sensors and sensing means embodying the invention are used to control the point in time at which power extracting circuits are switched in circuit with a power generating device. The power extracting circuit may include components which can resonate with the power generating circuit at a higher frequency than, and independent of, the frequency at which the power generating device is being operated. Thus, the electric power generator device operated and controlled by a slowly changing source of energy (e.g., ocean waves, wind, eddies of water) may develop energy at one frequency and may be operated to transfer the energy, at a selected point in time, at another frequency.
In certain embodiments where the power generator is capacitive, sensors are used to switch an inductive power extracting circuit on the positive and negative peaks of the input force. In accordance with the invention where a capacitive power generator produces an oscillatory electrical signal at a low first frequency (f1), there is switched into the system an inductive power extracting circuit designed to resonate with the capacitive power generator at a resonant frequency (fo), which is substantially greater than f1, on the positive and negative peaks of the input force such that power will be extracted in an electrical pulse which begins at switch closure and ends when the current reaches zero in the inductor. The time of switch closure, Tc, is equal to approximately 1/2 fo, where fo is the resonant frequency of the source and load circuit.
Sensors embodying the invention include circuits and systems for reliably and accurately detecting at least one of the peak(s), amplitude and frequency of an input force and circuitry for controlling the turn-on and turn-off of a switch selectively coupling a power extracting circuit to a power generating device.